It is well known that hydrogen is a clean and ideal energy source with high energy density. The industrialized production of nickel-metal-hydride alloy powder is one of the most important tasks of personnel specialized in the metallurgical industry. The industrialized production of the nickel-metal-hydride alloy involves the manufacturing technology of the nickel-metal-hydride alloy and the relevant facilities.
The related prior art is as follows:
The chemical compositions of AB.sub.5 type nickel-metal-hydride alloy in the prior art are almost the same, that is, they are obtained by modifications of LaNi.sub.5 nickel-metal-hydride alloy invented in Holland in 1968. Said modification lies in that a low-price mixture of rare earth (Mm) is used as a hydrogen adsorption element instead of high-purity metal La. Optionally, a small amount of Ti, Zr, Ca and Mg is added as hydrogen adsorption elements for further increasing the hydrogen adsorption capacity of the nickel-metal-hydride alloy; or a part of Ni is replaced by Co, Mn, Al, or 1 to 2 kinds of M elements (M represents V, Cr, Fe, W, Mo, Nb, B, Si, Sn, Zn, N and so on) are added for improving anti-corrosion, cycling life and related comprehensive properties of the nickel-metal-hydride alloy, so as to ensure the performance characteristics of Ni--MH battery, such as high capacity, long life, high reliability, and to reduce the raw material cost.
AB.sub.5 type nickel-metal-hydride alloy now has developed into an alloy almost the same as the MmNiCoMnAl series alloy. The ratio of hydrogen adsorption elements to non-hydrogen adsorption elements in AB.sub.5 type nickel-metal-hydride alloy is generally 1:5, which is properly adjusted by the manufacturers according to their own conditions, by increasing or reducing the contents of hydrogen adsorption elements. The atomic amount of non-hydrogen adsorption elements is generally 4.8-5.2. The contents of various other elements are commonly: Ni 1.5-3.5, Co 1.5-3.5, Mn 0.1-1, Al 0.05-0.5. A small amount of metal M can be added with a content of about 0.02-0.2.
There are mainly two types of mixed rare earth metal Mm, that is the Lanthanum-enriched type and Cerium-enriched type. The contents of the main rare earth elements of La, Ce, Nd, Pr vary according to different places of origin of the raw materials. Generally, the total amount of La and Ce is more than 70% so as to ensure the hydrogen adsorption capacity of the nickel-metal-hydride alloy.
In the state of the art, the manufacture of nickel-metal-hydride alloy mostly emphasized the use of Ni--MH battery for negative electrode. For improving the comprehensive properties of the electrode, the alloy composition is adjusted and the alloy structure is improved accordingly. However, the prior art rarely concern the industrial production method of nickel-metal-hydride alloy. The production methods of AB.sub.5 type nickel-metal-hydride alloy mentioned in the prior art can be summarized as follows:
1. Vacuum arc melting, casting and mechanical pulverizing method:
Mixed rare earth metal with high purity and proper particle size are blended metallic raw materials according to the proportion of alloy composition, and placed in a water cooling copper crucible, then evacuated and filled with Argon for arc melting. The above mixture needs to be melted several times for obtaining a homogeneous ingot, and then pulverized mechanically. Nickel-metal-hydride alloy is obtained by using this method in Japanese patent document nos. P3-289644, P4-52242 and P4-168240. The productivity of this method is low and can only be used in research work.
2. Vacuum induction melting, water cooled mould casting and mechanical crushing, pulverizing method:
As stated in Japanese patent document no. P3-188234, liquid metal, after being melted in a vacuum and optionally argon atmospheric induction furnace, is cooled quickly in plate water-cooled copper mould. A cast ingot having a column structure is obtained and then the mechanical crushing and pulverizing are used for powdering. The resulting mixture, under vacuum and optionally argon atmospheric protection, is heat treated at 900 to 1200.degree. C. for structural homogenization. In this method, it is difficult to control the stable quality of the cast ingot in the production of ingots of less than 10.mu. micro-crystal structure by using large capacity induction furnace.
3. Liquid metal single roller quick quenching and mechanical pulverization method:
Japanese patent document no. P2-301531 teaches that 5-15 mm blocks of AB.sub.5, AB.sub.2 and AB type metal-hydride alloy are made at first, then crushed coarsely and placed in a quartz tube for re-melting into liquid metal with high frequency induction heating under argon atmosphere. Then the resulting material is processed into strips by a high speed rotating (2,000 rpm) water cooled copper roller (.phi.300.times.400 mm), and fine powder is obtained by mechanical pulverization. Compared with other methods in the prior art, the quick quenching can improve the cooling speed, control the micro-structure of nickel-metal-hydride alloy, and increase the charge and discharge cycling life of the alloy powder.
4. Gas atomization method.
Japanese patent document no. P3-226408 teaches that AB.sub.5 type liquid nickel-metal-hydride alloy could be atomized by high speed inert gas for manufacturing non-balance state nickel-metal-hydride alloy powder. Thus, it is possible to increase the capacity, depress the self-discharge of Ni--MH battery and prolong the cycling life.
The Japanese patent No.P5-222474 discloses that the powder is obtained by the atomization of liquid AB.sub.5 type nickel-metal-hydride alloy MmNiCoMnAl+Zr under an Argon protection atmosphere. The cooling speed is &gt;500.degree. C./sec. An alloy with least segregation, micro-crystal and homogeneous structure can be obtained under homogenizing heat treatment at 600-900.degree. C. for 2-5 hours. The corrosion-resistant property of the alloy can be improved and cycling life can be extended. Said method is suitable to be used for manufacturing a negative electrode of high capacity with an initial capacity of 300 mAh/g and long cycling life (with a charge and discharge cycling life of &gt;500 times) Ni--MH battery. Said method can also omit the mechanical crushing and pulverization steps of casting an ingot and simplify the production process of nickel-metal-hydride alloy powder. However, it cannot satisfy the scale of industrial production.
5. High temperature reductive diffusion from rare earth oxide method.
In Japanese patent document no. P3-170601, high-purity rare earth oxide La.sub.2 O.sub.3 (purity 99.99%), high-purity nickel powder (purity 99.9%) of 5.0-8.8.mu., and reductive of high-purity Ca (purity 99%) are mixed homogeneously. The diffusion reaction is then proceeded in the reaction container at 970.degree. C. for 1.5-4 hours. LaNi.sub.5 alloy powder is obtained through cooling, water washing and filtering off the reaction product CaO, with a composition of La 31.4%, Ca 0.43-0.61%, O.sub.2 0.06-0.12%. The initial capacity of the alloy is 295-297 mah/g.
In Japanese patent document no. P3-281710, a mixed rare earth oxide (La.sub.2 O.sub.3 50.63/27.61, CeO.sub.2 2.81/50.1, Pr.sub.6 O.sub.11 /4.32, Sm.sub.2 O.sub.3 0.20/0.10), Ni powder, and CoMmCuAl metal oxide powder are blended homogeneously according to the chemical equivalent proportions of the nickel-metal-hydride alloy, with a small amount of Ca, Mg or Li being added. After mixing, MmNiCoMnAl alloy can be obtained by a high temperature reductive diffusion method, wherein the reaction temperature is 1,000-1,200.degree. C., and the reaction time is 4-6 hours. Combining the rare earth oxide reduction, alloying and pulverization into one step can save the steps of mechanical crushing and pulverization of a cast ingot, improve the heat conductivity, electric conductivity, and reduce the danger of pulverization by hydrogenation. But, the technology is complex and can not be used in continuous production and cannot satisfy the requirement of large-scale production.
Except the aforementioned manufacturing methods, the methods and equipment for further homogeneous treatment and hydrogen heat treatment, especially the facilities for hydrogen heat treatment, of nickel-metal-hydride alloy powder are rarely reported. After studying, the applicants find that the further homogeneous treatment and hydrogen heat treatment of nickel-metal-hydride alloy powder can obtain a micro-crystal and homogeneous alloy structure with little segregation in composition, increased thermo-resistance and extended charge and discharge cycling life.
This invention is particularly developed for overcoming the shortcomings present in the existing method and equipment for manufacturing of nickel-metal-hydride alloy powder.